This invention relates to control of the volume of air flowing through a duct. In particular, the invention concerns a method and apparatus for reducing the damper-closing torque in an air flow controller which results from aerodynamic forces acting on the damper so as to permit accurate damper position control using a relatively small actuator.
It is well known to control air flow through a duct using a betterfly damper mounted within the duct and pivoted near its midpoint. Typically, an actuator is employed to position the damper between open and closed positions in response to a sensed condition. An air flow control of this type is disclosed in U.S. Pat. No. 3,809,314 issued to Roger P. Engelke and Marvin H. Zille.
As is well known, a damper inclined to the air flow in a duct is subjected to aerodynamic forces similar to the lift and drag forces which act on an airfoil. These aerodynamic forces produce a torque which tends to close the damper. Because of the presence of this torque, the damper position control or actuator must act in opposition to the torque and must be able to withstand the effects of this torque in order to maintain the damper in the desired position. This situation necessarily limits the amount of reduction in actuator size and power consumption which can be achieved while maintaining accurate damper positioning capabilities.